To answer your question, it would be important to know what materials the sound wave is moving through on each side of point X. However, based on typical behavior of sound waves when they transition between different materials, the likely scenario is:
C. The sound wave has moved into a new material and slowed down.
This choice is appropriate if the sound wave is moving from a less dense medium (like air) to a denser medium (like water). Conversely, if the wave is moving from a denser medium to a less dense one, it could speed up.
If you can provide more context about the specific materials involved, I would be happy to provide a more accurate answer!